"tranquility, peacefulness, quietness," 1610s, from Latin placiditatem (nominative placiditas), from placidus "peaceful, quiet, gentle, calm" (see placid).
Entries linking to placidity
placid adj.
"gentle, quiet, undisturbed, serene, calm," 1620s, from French placide (15c.) and directly from Latin placidus "pleasing, peaceful, quiet, gentle, still, calm," from placere "to please" (see please). Related: Placidly; placidness.